The present invention relates to an apparatus and method for detection of plaque and is related to an apparatus and method that is useful to non-invasively locate and identify vulnerable plaque. The present invention finds application in conjunction with a diagnostic imaging system having a nuclear diagnostic imaging sub-system in combination with an x-ray imaging sub-system and will be described with particular respect thereto.
Atherosclerotic cardiovascular disease, especially some aspects of coronary heart disease (CHD), has been thought to develop gradually. In recent years, it has become known that occlusion may occur suddenly, potentially resulting in thrombus formation and angina, myocardial infarction (MI) or sudden death. This shift in study of one operative mechanism for CHD is related to evolving understanding of different types of plaque in coronary arteries that may cause acute coronary syndromes. Present study of plaque in CHD involves identification, characterization and location of plaque including, more specifically, the roles of stabilized plaque and vulnerable plaque in CHD.
Stable plaque is characterized as having a fibrous rich cap over a liquid core. A vulnerable plaque, which may be more likely to erode or suddenly and unpredictably rupture, is less fibrous, has less muscularity, more lipids and inflammatory cells. Particularly vulnerable plaque is characterized as having a thin fibrous coating over a large, lipid rich core that contains numerous inflammatory cells. Vulnerable plaque includes various high-risk plaques thereby predisposing patients to develop acute thrombotic coronary syndrome.
The atherosclerotic plaques are not merely an accumulation of cholesterol on the artery wall. They include an active collection of different migrated, proliferated and infiltrated cells such as mainly smooth muscle cells and immune cells along with one or more immune-triggering agents such as oxidized LDL, infectious agents, heat shock proteins as well as other factors. These metabolically active vulnerable plaques can remain quiescent and sub-clinical for years then erode or suddenly erupt with an associated deleterious effect on the cardiac system.
Failure of present methods to identify future acute coronary syndromes has led to a search for more effective diagnostic techniques. One example of a present diagnostic technique that is not as successful as desired is the failure of coronary angiography to predict catastrophic cardiovascular failure. One reason for this is vulnerable plaques are usually angiographically non-significant.
Some techniques that are under study to identify, locate and characterize plaque in the coronary arteries include invasive and non-invasive methods. At present, the clinically available techniques tend to have disadvantages that limit their use in asymptomatic patients or are not as effective as desired in identifying, locating and characterizing vulnerable plaque which are believed to represent the higher risk of fatal CHD.
Some examples of invasive techniques presently under evaluation include angiography, Intravascular Ultrasound, Intravascular Thermography, Optical Coherence Tomography, Intravascular Electrical Impedance Imaging, Photonic Spectroscopy, as well as a number of other Spectroscopy techniques. Conventional non-invasive techniques being studied for plaque identification, location and characterization include MRI (including contrast media), Electron Beam CT, multi-slice and spiral CT as well as conventional Nuclear imaging.
Even with such diverse diagnostic techniques under development and study for the early identification of vulnerable plaque, the degree of clinical effectiveness is presently less than desired in a clinically practical setting. One reason is that some of these imaging modalities primarily provide anatomical information and, given the composition and metabolic activity associated with vulnerable plaque, functional information is particularly useful in determining whether a plaque deposit is stable or vulnerable. However, eventhough conventional nuclear medicine imaging techniques can provide functional images, the total volume of vulnerable plaque is extremely small and the total plaque uptake of tracers used in nuclear medical imaging is somewhat limited. Consequently, the application of conventional nuclear medicine clinical techniques using SPECT and PET for plaque detection has not yet provided a clinically acceptable technique that generates clinically practical diagnostic results. More specifically, when considering typical plaque deposits and tracer uptake amounts, the small volume of the plaque deposits and limited uptake of tracers result in low count acquisition rates. In order to obtain clinically useful images with such low count acquisition rates, scanning time would need to be increased perhaps 40 fold over that of a typical myocardial perfusion scan. This scale of increase in scanning time is not clinically practical.
In addition, other risks associated with the invasive techniques may make them less desirable for office based clinical application, particularly for the asymptomatc patient population. Diagnostic imaging systems such as MRI or CT may be cost prohibitive in an office based practice. Present clinical techniques for conventional full application nuclear medicine systems (SPECT and PET) have clinically unacceptable scanning time resulting from count rate limitations as discussed above. Furthermore, many available full feature tomographic nuclear medicine systems may have cost and space considerations making them less desirable for office based clinical screening of patients.
It is desirable to have an apparatus and method for a less expensive, compact, non-invasive, clinically practical diagnostic imaging technique that provides anatomical and functional information to identify, locate and characterize vulnerable plaque for widely used preliminary screening and early detection of CHD in the general population. It is also desirable to have a system that provides high resolution, relatively large field of view and good clinical sensitivity in an office based clinical setting.